Plug-assist vacuum thermoforming, in which generally concave, hollow articles such as plastic cups are formed from sheets of plastic material, is well known in the art. In such processes, a sheet of thermoplastic resin is softened by heating, clamped in a frame, and then a tapered, blunt-nosed plug member is used to push the sheet down into the die, usually from 60% to 95% of the way in. Then, vacuum is applied through pores in the die surface to pull the sheet off the plug and down against the die surface. The plug is usually heated to produce slipping of the softened sheet over the surface of the plug as the plug stretches the sheet. A problem occurs when the sheet is of a material which will not slip well across the plug, and when a relatively deep item is to be formed, resulting in portions of the sheet being drawn thin while other portions, particularly on the bottom of the plug, remain at their original thickness. As a result, a thick section is formed at the bottom of the article, and the sides of the article, particularly where they join the bottom, are relatively thin, resulting in structural weakness. This variation in thickness becomes particularly objectionable when forming premium, heavy-weight plastic cups.
Further, even in plastic cups of typical thickness, an increased amount of material must be used to maintain minimum strength levels to overcome these thickness variations produced by the forming process. The rigidity of a plastic cup depends largely on the thickness of the side wall. Thus, if excess material weight could be removed from the bottom of the cup and relocated into the side walls, overall material savings and weight reduction might be realized while maintaining the same rigidity.
One early effort to address this problem is disclosed in U.S. Pat. No. 2,990,581 to Rowe, Jr. which shows a plug assisted vacuum thermoforming process wherein the plug has a deep cone-shaped central well as shown in FIGS. 2 and 3 of that patent. In one embodiment, a port at the apex of the cone supplies a vacuum during the descent of the plug into the forming die to "predraw" the central portion of the sheet which will form the bottom of the thermoformed article. The Rowe device does not disclose using air pressure to predraw any other regions of the sheet, such as the areas forming the sides of a cup, and the Rowe plug port does not perform any material ejection function.
U.S. Pat. No. 3,814,784 to Wolf discloses a plug assisted thermoforming method and a plug which includes a passage connected to a source of reduced and superatmospheric pressure. When the plug is connected to the reduced pressure source, the plastic sheet being thermoformed is pulled against the plug. The plug disclosed by this patent does not have a central well, but instead has a substantially rectangular configuration. Moreover, the plug does not prestretch the plastic sheet in this process.
U.S. Pat. No. 2,973,558 to Stratton, Jr. and U.S. Pat. No. 4,443,401 to Turner disclose plug assist methods for thermoforming containers in which the plastic is pre-stretched prior to its introduction into the mold. In both these methods, however, the sheet is stretched only by pressurized air directed from the mold. A vacuum is not applied to the plug to produce the pre-stretching.
U.S. Pat. No. 3,342,914 to Edwards discloses a porous mandrel or plug (FIGS. 10-13) through which a vacuum is broadly applied to pull a plastic sheet to the shape of the plug while it is being directed into a corresponding shaped mold. The plastic is not pre-stretched by the tip of the plug prior to directing it into the mold.
U.S. Pat. No. 3,441,983 to Cheney discloses the controlled pre-stretching of a plastic sheet by the application of positive and negative pressure air to produce a container with a uniform wall thickness, but does not disclose a plug-assisted method or a concentric wave prestretching pattern.
However, prior art plugs and plug-assisted methods of the types shown in these documents did not provide a fully optimal uniform distribution of material in formed articles, and further did not provide optimal processing speed. Therefore, there is a need for an improved plug and plug-assisted process for thermoforming of hollow articles.